This invention relates to a device for automatically lapping valve seats and more particularly to such a device suitable for repairing wedge-shaped gate valve seats and the like used in atmospheres highly contaminated with radioactive substances in nuclear power plants or the like.
Nuclear power plants are subject to legal periodic inspections at predetermined time intervals. Under these circumstances it is frequently required to lap valve seats in order to tailor the particular plant to such an inspection or upon repairing wedge-shaped gate valve seats and other valve seats as a result of the inspection. The lapping operation has been previously necessary to be manually performed. Therefore only after the radiation level within the particular nuclear reactor has been reduced to its permissible value or less, are repair personnel permitted to enter that nuclear reactor to perform the operation of lapping valve seats, and then only for a time period as short as possible. It has been previously said that the radiation dose received in each operation is inversely proportional to the square of the distance from a contamination source involved and also such a radiation dose is proportional to the working hours. Therefore the same operator can not work for a long time and the exposure dose of the operator restricts his working hours. This has unavoidably led to the shift of repair personnel afer short working hours for the control of their health and accordingly to the necessity of securing much skilled labor.
On the other hand, devices for lapping valve seats have been previously proposed. Most of the proposed lapping devices have been of the manually operated type although some of the devices have been of the power operated type. In either type of conventional lapping devices a valve seat to be lapped has been coated with a lapping agent and centered on the lapping plate. Then the lapping plate has been rotated with a predetermined rotational force with respect to the valve seat thereby to lap the latter by the lapping agent. However those devices have been of such a structure that the pressure applied to the interface of the valve seat and the lapping plate does not remain unchanged and also can not be adjusted in accordance with the area of the particular valve seat and for each cycle of the lapping operation the lapping agent is supplied to the valve seat being lapped or the next valve seat after the removal of the lapping plate. It is desirable to provide a device for automatically lapping valve seats operated from a remote position. This is particularly desirable for lapping valve seats used in a nuclear reactor because jobs performed within the reactor are not desirable from the standpoint of the health of the operators.